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CodingBat doesn't always point you in the best direction. Rather than dealing with the min and max methods for class, you could just use the sort method for arrays.
int[] nums = {2,7,9,1};
Arrays.sort(nums);
return nums[nums.length - 1] - nums[0];

Try printing the values of large and small at the end of each iteration of the loops and see how they change as your code processes one of the tests. You should be able to pick out what's going wrong pretty quickly.

You seem to have posted eight Challenge questions in 40 minutes.
It doesn't seem like you are giving these questions any thought at all.
Why don't you try one at a time and see if you can actually learn something??